Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 500		$ 396
Asset Retirement Obligation, Accretion Expense	5	[1]
Asset Retirement Obligation, Revision of Estimate	99	[2]
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	493		389
Asset Retirement Obligation, Accretion Expense	5	[1]
Asset Retirement Obligation, Revision of Estimate	99	[2]
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	7	[3]	7	[3]
Asset Retirement Obligation, Accretion Expense	1	[1],[4]
Asset Retirement Obligation, Revision of Estimate	$ 1	[2],[4]

[1] Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

[2] The ARO increase also resulted in a corresponding increase recorded to “Property and Plant, Net.” Ameren Missouri’s estimates related to its Callaway energy center decommissioning costs changed to reflect increased costs from the 2015 cost study and funding analysis, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information

[3] Included in “Other deferred credits and liabilities” on the balance sheet

[4] Less than $1 million.